The NUN protein is expressed by E-coli after infection with lambdoid phage HK022. The presence of the protein terminates the transcription of DNA and prevents superinfection of E-coli by other lambda phages. From a tryptic digest of the NUN-RNA complex it was determined using MALDI-TOF mass spectrometry the interaction region was traced to a 30 residue stretch of the protein. More detailed mapping of the binding region is being carried out through the use of a ladder of peptides that span the region of interest.